In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and UEs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying user data from the base station to UEs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to UEs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying user data from UEs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from UEs to the base station.
In LTE, uplink and downlink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines a continuum of 10 millisecond (ms) frames, divided into 1 ms sub-frames and 0.5 ms slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
In LTE, uplink and downlink resources are typically allocated to individual UEs in blocks that may referred to as resource blocks. In the time domain, each resource block has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a resource block accommodates 14 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference.
The smallest unit of air interface resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Most of the resource elements in each resource block are available to carry user data. However, some of the resource elements are reserved for certain functions. For example, in each downlink resource block, a certain number of resource elements (e.g., 8 resource elements) are used to transmit reference signals, and a certain number of resource elements (e.g., resource elements in the first one to four OFDM symbols) are used for control channels.
Each resource element that carries user data provides a certain number of information bits, depending on the modulation and coding scheme (MCS) that is used. Each type of MCS includes a type of modulation and a type of coding that adds bits for error correction. The modulation determines the number of bits represented by each resource element. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each resource element represents 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each resource element represents 4 bits; and with 64 Quadrature Amplitude Modulation (64QAM), each resource element represents 6 bits. However, the number of information bits provided by a resource element is typically less than the number of bits represented by a resource element because of the additional bits that are added for error correction. For example, with 64QAM, each resource element represents 6 bits. However, the number of information bits provided by each resource element may range from about 2.730 to about 5.555, depending on the error correction coding that is provided by different MCSs. The number of information bits per resource element that is provided by an MCS is often described as the efficiency of the MCS.
A typical LTE implementation supports a range of MCSs with different efficiencies. For example, the supported MCSs may include MCSs that use QPSK modulation, with efficiencies ranging from about 0.152 to about 1.176, MCSs that use 16QAM modulation, with efficiencies ranging from about 1.477 to about 2.406, and MCSs that use 64QAM modulation, with efficiencies ranging from about 2.730 to about 5.555.
Typically, an MCS is selected for a resource block based on channel conditions. In general, higher-efficiency MCSs are selected when channel conditions are good and lower-efficiency MCSs are selected when channel conditions are poor. More particularly, the signal-to-noise ratio corresponding to particular channel conditions may result in different average error rates for different MCSs. To select an MCS for the particular channel conditions, the average error rates of the MCSs may be compared to a maximum acceptable error rate (i.e., an error rate that is deemed acceptable). The MCS with the highest efficiency that results in an average error rate that is less than or equal to the maximum acceptable error rate may then be selected.